In a drum type washing machine, an approximately cylindrical drum having a circumferential wall with a large number of perforations is mounted on a horizontal or inclined rotation shaft. In the dehydrating operation, the drum is rotated at high speeds so that the laundry contained in the drum is squeezed and the water held therein is removed. In this type of washing machine, an uneven distribution of the laundry on the inner circumferential wall will cause a mass imbalance (i.e. an eccentric load) around the rotation shaft. In this state, the drum strongly vibrates if it is rotated at a high speed. The vibration of the drum leads to a vibration of the outer tub enclosing the drum. The outer tub in turn collides with the inside of the housing and causes it to vibrate. As a result, an abnormal vibration and noise occurs. Therefore, for drum type washing machines, it is very important to reduce the vibration and noise during the rotary dehydrating operation.
In drum type washing machines, if an adequate amount of laundry articles are contained in the drum, it is often possible to reduce the eccentric load of the drum by appropriately distributing the laundry articles along the inner circumferential wall of the drum. For this purpose, it is necessary to loosen the laundry articles because they are usually tangled with each other after the washing or rinsing process. Therefore, in a conventional drum type washing machine, a specific kind of operation, called the agitating operation, is carried out, in which the drum motor is controlled so that the drum rotates at low speeds of about 40 to 60 r.p.m. to agitate and loosen the laundry articles for a predetermined period of time and subsequently the drum speed is raised to a level at which the laundry articles are pressed on the inner circumferential wall of the drum by centrifugal force.
However, the above-described conventional control method does not always ensure that the drum speed is raised at a good timing when the laundry articles are adequately loosened and easy to distribute. Therefore, the possibility of appropriately distributing the laundry articles into a state where the eccentric load is adequately small is weak. If the eccentric load is still large when the drum speed is raised, it is necessary to lower the speed to carry out the agitating operation again and then raise the speed. Repeating such a process often requires a very long time until the dehydration process is actually started.
A conventional method for determining an appropriate timing for changing the drum speed from a low level for loosening the laundry to a high level is known from the Japanese Unexamined Patent Publication No. H8-876. The drum type washing machine disclosed in this Publication includes a vibration sensor for measuring the amplitude of the vibration of the outer tub. The sensor is used for detecting an abnormal vibration of the outer tub while the drum is rotated at a low speed for loosening the laundry. If no abnormal vibration is detected, the drum speed is raised, whereas the loosening operation is continued in the case an abnormal vibration has been detected. This control method prevents the drum speed from being unnecessarily raised when the laundry is still in a mass form that is hard to distribute. Usually, however, once the outer tub starts a strong vibration due to an uneven distribution of the laundry during the low speed rotation of the drum, the vibration will not immediately fade away even after the laundry is adequately distributed. Therefore, it is necessary to wait for a certain period of time until the next detection of the vibration. This means that it is impractical to continuously check and determine the massing or gathering state of the laundry during the low speed rotation of the drum.
Except for the case where the laundry articles are tightly tangled, the state of the laundry continuously changes with time during the low speed rotation of the drum. For example, even if the laundry articles are relatively loosened and moving separately inside the rotating drum at a certain point in time, they may soon be in a different state where plural pieces of laundry articles are overlapped and gathered in the form of a mass that rolls or falls without being separated. Therefore, it is essential to continuously check the state of the laundry contained in the drum, otherwise the drum speed cannot be raised at the best timing where the laundry is adequately distributed. For this reason, the aforementioned conventional washing machine often fails to detect the best timing for increasing the drum speed and resultantly takes a long time for the loosening operation.
In the case the laundry is tightly tangled in a mass form, there is only a small chance that the laundry articles will be loosened by continuing the loosening operation for a long period of time. However, the aforementioned conventional washing machine cannot distinguish the case where the laundry is in the form of an extremely tight mass from the case where the laundry is a simple pile of laundry articles that are not tightly tangled. As a result, the loosening operation often continues even in the former case where the loosening effect is least expected, consuming an unnecessarily long period of time until the start of the dehydration process.
To address the above-described problems, the present invention aims to provide a drum type washing machine capable of efficiently distributing the laundry articles to reduce the eccentric load in the initial phase of the dehydration process, so that not only the time consumed until the dehydrating operation but also the total operation time are shortened.